1. Field of the Invention
The present invention relates to an image pickup apparatus having a lens holding unit and an adjustment mechanism, which is suitably used as an image pickup apparatus such as a copying machine designed to perform image formation by forming original images, illuminated with light through a slit, on the surface of an image carrier, e.g., a photosensitive member, at various magnifications by moving an imaging lens (zoom lens) on the optical axis using, e.g., a variable magnification mechanism.
2. Related Background Art
A zoom lens has been conventionally used as an imaging lens for an image pickup apparatus such as a copying machine having a variable magnification mechanism. A zoom lens of this type is disclosed, for example, in Japanese Laid-Open Patent Application No. 57-67909.
In this official gazette, a so-called four-group zoom lens is disclosed, which comprises a first group as a stationary group, a second group as a movable group, a stop, a third group as a movable group, and a fourth group as a stationary group which are sequentially arranged from the object side (original surface side). The first group is constituted by one negative lens and has negative refracting power. The second group is constituted by three lenses, i.e., a positive lens, a negative lens, and a positive lens, and has positive refracting power. The third group is constituted by three lenses, i.e., a positive lens, a negative lens, a positive lens, and has positive refracting power. The fourth group is constituted by one negative lens and has negative refracting power. These four groups are arranged to be substantially symmetrical about the stop. The magnification of this zoom lens is continuously changed by moving the second and third groups on the optical axis while moving the overall lens system.
In such a four-group zoom lens, changes in image formation characteristics due to deviations from set values of, e.g., the lens surface precision, the lens thickness, and the refractive index of a lens material, an assembly error of each constituent element, and the like are corrected by moving the second and third groups as movable groups in the direction of the optical axis to finely adjust the focal length.
FIG. 1 is a sectional view showing a main part of a copying machine as an electrophotographic copying machine of a slit exposure scheme. Referring to FIG. 1, reflected light 2 from an original 1 illuminated with a beam of light from a light source 5 through a slit is reflected by mirrors M1, M2, and M3 to be focused on a photosensitive drum 4 by a zoom lens 3 through mirrors M4, M5, and M6. The original 1 is sequentially moved/scanned, from an end portion a to an end portion b, at the same speed. At the same time, the photosensitive drum 4 is rotated to perform development/transfer of the original image by a known electrophotographic process, thus visualizing the image on a transfer sheet.
In an image pickup apparatus having a variable magnification mechanism, the distance from the surface of the original 1 to that of the photosensitive drum 4, i.e., the object-image distance, is generally kept constant by using a zoom lens as an imaging lens, regardless of the image forming magnification.
When a one-to-one image of the original 1 is to be obtained, the zoom lens 3 is optically set at an almost equal distance from the original 1 and the surface of the photosensitive drum 4, as shown in FIG. 2. When an enlarged image of the original 1 is to be obtained, the zoom lens 3 is moved toward the original 1 side, and the focal length of the zoom lens 3 is changed to a value corresponding to the image forming magnification of the image, as shown in FIG. 3.
When a reduced image of the original 1 is to be obtained, the zoom lens 3 is moved toward the photosensitive drum 4, and the focal length of the zoom lens 3 is changed to a value corresponding to the image forming magnification of the image, as shown in FIG. 4.
FIGS. 5 and 6 are sectional and front views, respectively, showing a main part of a lens holding unit for holding lenses constituting the zoom lens 3.
A guide rail 9 and a cam groove plate 10 are arranged on a lens mount 7 of the copying machine. The guide rail 9 serves to guide an overall lens unit 8. The cam groove plate 10 serves to guide a cam plate 11. When an enlarged or reduced image is to be obtained, the lens unit 8 is moved by a drive source such as a motor (not shown) by a distance corresponding to a designated image forming magnification while being guided by the guide rail 9.
Cam grooves 12 and 13 are formed in a middle portion of the cam plate 11. Two protruding pins 14 protruding from lower portions of two lens barrels B are respectively coupled to the cam grooves 12 and 13. The two lens barrels B are guided by elongated holes 15 formed in the direction of the optical axis of a lens barrel A so that the lens barrels B are moved in the direction of the optical axis in accordance with the shapes of the cam grooves 12 and 13 upon movement of the cam plate 11.
The shapes of the cam grooves 12 and 13 are determined in accordance with the image forming magnification of the zoom lens 3, and the like. Lenses G2, G3, and G4, and lenses G5, G6, and G7 are respectively held/fixed in the two lens barrels B, and lenses G1 and G8 are held/fixed in the lens barrel A. A stop plate 16 for restricting a beam of light is arranged between the lens barrels B.
When the lens unit 8 is moved to a designated magnification position by the drive source while being guided by the guide rail 9, the lens barrels B move along the cam plate 11 and the cam groove plate 10 and are guided by the cam grooves 12 and 13 of the cam plate 11 to move through the lens barrel A in the direction of the optical axis, thus changing the focal length of the lens unit 8.
With this operation, a conjugate relationship is established between the surface of the original 1 and that of the photosensitive drum 4 with respect to each image forming magnification, thereby obtaining an enlarged or reduced image. The shapes of lenses G1 and G8, and those of the lenses G2 to G4, and G5 to G7 are symmetrical about the stop plate 16. In this arrangement, the two lens barrels B are assembled in the lens barrel A such that one of the lens barrels B is reversed laterally with respect to the other.
With a reduction in space for a copying machine, demands have recently arisen for a zoom lens which can realize a reduction in the size of the overall lens system and an increase in field angle. In addition, with an improvement in the performance of a copying machine, demands have arisen for a high-magnification, high-performance lens system.
In general, in order to realize a zoom lens which can satisfy such demands, various requirements must be satisfied. For example, in order to reduce the size of the lens system, the refracting power of each lens group must be increased. In order to properly compensate for the optical performance, the processing precision of each lens, the processing precision of each lens barrel, and the like must be increased. If, for example, the processing precision of each lens is insufficient, the focal length of the overall lens system varies because of variations in the lens surface precision, the lens thickness, and the refractive index of a lens material, thus increasing various aberrations, e.g., a curvature of field. As a result, the image formation performance cannot be properly maintained throughout the variable magnification range and the image plane.
The lens barrel structure of a zoom lens used for a copying machine is constituted by a lens system which is symmetrical about a stop. As shown in FIG. 5, part of the lens barrel structure is laterally symmetrical.
For this reason, if the axial position of the lens holding portion of the lens barrel is offset from the central axis of the guide portion of a moving lens barrel by a distance .DELTA.x, as shown in FIG. 7, an aberration caused at the front group is increased, resulting in a considerable deterioration in image formation performance.
Assume that the height of a lens unit is decreased by reducing the thickness of a lens barrel portion in order to realize a reduction in the size of the overall lens system with a reduction in space for a copying machine. In this case, the lens barrel is deformed by a pressure applied upon mounting of a lens press ring, or a shock produced in an assembly operation. As a result, the lens groups are inclined to cause an asymmetrical curvature of field, thus deteriorating image formation performance.
The arrangement for directly pressing a lens to adjust the lens position in the lens barrel especially produces strain in the lens barrel and the lens. As a result, the holding position and the optical axis of the lens are decentered to greatly deteriorate optical performance.